Microsoft has issued a warning in the knowledge base article for the MS14-066 update released this past week. The company has provided a workaround, but is not recommending that users avoid the update or uninstall it.
The update fixed at least one critical vulnerability in Schannel, Microsoft's implementation of SSL/TLS encryption. It has widely been considered highly critical and last week we urged users to apply the update as soon as possible.
But some users who apply the update are having serious problems. The issues occur in configurations in which TLS 1.2 is enabled by default and negotiations fail. When this happens, according to Microsoft, "TLS 1.2 connections are dropped, processes hang (stop responding), or services become intermittently unresponsive." There may also be an event ID 36887 in the System event log withe description "A fatal alert was received from the remote endpoint. The TLS protocol defined fatal alert code is 40."